This invention relates to optical disk play-back systems, and more particularly to center-error detection for a lens and control for a sledge in the optical pickup.
Optical disks such as compact disk (CD) and digital-versatile disk (DVD) have been quite popular in computer and consumer devices. Data is stored on the disk by pits on the surface or on an internal layer within the disk. When a laser or other light source is shined on the spinning disk, the light is scattered by the pits but reflected back to a photosensor when no pit is present.
The pits are arranged on the disk into a continuous spiral track. The laser and pickup head can be moved along the radius of the disk to select different portions of the continuous spiral track. Indeed, along any radius of the disk, the disk appears to have many separate tracks of increasing distance from the first track at the inner diameter (ID) to the last track at the outer diameter (OD) of the disk.
FIG. 1 shows a pickup assembly for an optical disk. During operation, optical disk 98 is mounted on spindle 92, which fits into the hole at the center of optical disk 98. Spindle 92 has a motor (not shown) that spins the disk. Spindle 92 is mounted on or formed in pickup frame 100.
Pickup frame 100 is a frame around a hollow center. Sledge 90 slides along rods 94, which are metal rods mounted to pickup frame 100. Sledge 90 slides along the radius of optical disk 98 mounted on spindle 92, allowing any track on optical disk 98 to be selected for reading.
Sledge 90 contains electronics and optics for reading optical disk 98. Lens 12 receives a laser beam reflected from the surface of optical disk 98 and focuses the light onto photodiodes 10. A prism (not shown) is used to bend the light beam by ninety degrees.
Lens 12 fits inside cavity 96 on sledge 90. As optical disk 98 is being read, the position of lens 12 within cavity 96 is altered slightly by a tracking control loop. The signals from photodiodes 10 are used by the electronics in the tracking control loop to adjust the position of lens 12 within cavity 96 to keep lens 12 on the track. The rapid rotational speed of optical disk 98 causes the tracking adjustment signals to have a high frequency, allowing the position of lens 12 to be adjusted quickly. A focusing control loop also uses the signals from photodiodes 10 to rapidly adjust the vertical distance that lens 12 is above the reading surface of optical disk 98.
Since the tracks on optical disk 98 are actually one long spiral track, as optical disk 98 is continuously read, lens 12 gradually shifts to the outer diameter of optical disk 98, toward the left of FIG. 1. Eventually lens 12 would reach the left edge of cavity 96 if sledge 90 did not move. Motor 21 uses gears 13 which engage sledge 90, moving sledge 90 to also follow the track being read on optical disk 98.
In prior-art CD readers, motor 21 could simply be driven by a counter or other periodic signal, causing sledge 90 to gradually move to the OD at a constant rate. The tracking motion of lens 12 within cavity 96 was sufficiently large to maintain tracking with sledge 90 moving at a constant rate. The rate could be adjusted as the rotation speed of spindle 92 changed, or when an error in reading occurred.
However, newer DVD optical disks have a much higher recording density. Over 40,000 tracks are present from ID to OD on a DVD disk. Cavity 96 limits the tracking motion of lens 12 to about 500 tracks. Thus an additional control loop is always used to control motor 21, adjusting the position of sledge 90 more precisely. This is known as a center-error (CE) control loop.
FIG. 2 illustrates a photodiode pickup moving among tracks on an optical disk. Photodiodes 10 contains four photodiodes, once for each quadrant. When photodiodes are exactly lined up along a track, photodiodes A and D pickup the same signal as photodiodes B and C. When mis-aligned, photodiodes A, D pickup more or less signal than photodiodes B,C. The difference in signal can be used to adjust the tracking, moving the lens focused onto photodiodes 10 either toward the outer diameter (OD) of the disk or toward the inner diameter (ID) of the disk.
Since any track on the disk can be selected, the sledge motion is rather large, spanning most of the radius of the disk. In contrast, the tracking motion of the lens within the cavity of the sledge is rather limited. High-frequency tracking-error circuits are used to rapidly adjust the lens position. Such tracking error generators using quadrant photodiodes are common. See for example, U.S. Pat. No. 5,859,816 by Yamamoto, and assigned to Toshiba, which uses phase comparison method. Focus control, where the optical distance to the disk is altered, is also known, such as described by Shimizume et al. in U.S. Pat. No. 5,475,664, and assigned to Sony Corp.
Another error is introduced when the lens is moved within the cavity by the tracking-error control loop. The lens position must be continuously adjusted within the cavity to maintain optical alignment with the track being read. However, if the sledge motion does not exactly match the track position, the lens may not be directly over the track. The light beam deviates from the perpendicular and a parallax error occurs. When the track is not directly under the lens, the alignment of the lens relative to the photodiodes and the disk surface is skewed from the perpendicular.
This parallax or center error can be corrected by shifting the position of the lens back to the center of the cavity in the sledge. However, the position of the sledge must change so that optical alignment to the track can be maintained. Sledge-centering errors are usually ignored in CD""s since the larger track spacing and lower recording density make the error relatively insignificant. However, the sledge-centering error is more significant for higher-density DVD optical disks.
The lens may be shifted in position by a small tracking motion relative to the rest of the photodiode pickup assembly once the sledge is repositioned over the track being read. The range of motion of the sledge must be large, since during seek operations, the gears must quickly move the sledge to the desired track. During a continuous read operation, this sledge movement is very slight because the track spacing is very small. FIG. 2 shows the tracking movement of the lens within the sledge""s cavity as being slight, while overall sledge movement is large.
FIG. 3 shows a feedback loop that detects a sledge-center error and shifts the sledge position, allowing the tracking loop to shift the relative position of the lens within the sledge. Lens 12 receives a light beam reflected from a recording layer within an optical disk. Lens 12 projects this beam to photodiodes 10. The four photodiodes 10 each send a signal to preamplifier 11, which amplifies the four signals A, B, C, D from the four photodiodes 10. These four amplified signals are input to center-error detector 15. Center-error detector 15 compares the relative signal strengths and generates a center-error signal CE.
The center-error signal from detector 15 is amplified by amplifier 17 before being input to motor driver 19. Motor driver 19 controls motor 21. The output of motor 21 is geared down by gears 13 and then controls the position of sledge 18 that holds lens 12. As the position of sledge 18 is changed, a tracking control loop (not shown) rapidly changes the position of lens 12 Motor 21 thus controls the position of sledge 18. The relative position of lens 12 to photodiodes 10 is then adjusted by the tracking loop.
As the relative position of lens 12 within sledge 18 changes, the signal strengths from photodiodes 10 changes. This change is amplified and compared to generate the center-error signal CE that controls motor 21 and the position of sledge 18. The tracking feedback loop then adjusts the lens position to minimize the center error.
The center error is a low-frequency signal, since the motion of sledge 18 is slow. The tracking error that controls the position of lens 12 within the cavity on sledge 18 is a high-frequency signal, rapidly moving the much lighter lens to maintain tracking. Extracting the low-frequency sledge-center error from the photodiodes is difficult, since the high-frequency tracking error can dominate the photodiode signals. Without sledge-center error control, the lens could reach the edge of the cavity in the sledge, preventing further tracking adjustments. Such tracking-error saturation could cause the tracking to be lost, requiring a lengthy re-read of the data.
FIGS. 4A-4C highlight sledge centering errors. In FIG. 4A, lens 12 is exactly centered within the sledge that includes photodiodes 10. No center error exists. Photodiodes A and D receive light reflected from region 16 on the optical disk, while photodiodes B and C receive the light reflected from region 14 on the disk. Since the middle of photodiodes 10 is aligned with the middle of lens 12, the angle that light reflects from region 16 of the disk to photodiodes A, D has the same magnitude as the light reflected from region 14 to photodiodes B, C. Regions 14, 16 have the same area and angle of reflection. Thus the intensity of the light received by photodiodes A, D is the same as the intensity of light received by photodiodes B, C, on the average. This can be expressed as (A+D)=(B+C), and the center error CE=[(A+D)xe2x88x92(B+C)]=0.
In FIG. 4B, lens 12 is mis-aligned to the outside (OD) of the cavity within the sledge that includes photodiodes 10. A negative center error exists. Photodiodes A and D receive light reflected from region 16 on the optical disk, while photodiodes B and C receive the light reflected from region 14 on the disk. The middle of photodiodes 10 is shifted toward the center of the optical disk compared with the middle of lens 12.
The angle that light reflects from region 16 of the disk to photodiodes A, D is lower or more glancing when compared with the light reflected from region 14 to photodiodes B, C, which has a larger angle of reflection (is closer to the perpendicular). Region 16 has a larger area than region 14, so that the light intensity reflected from region 16 is reduced. Thus the intensity of the light received by photodiodes A, D is less than the intensity of light received by photodiodes B, C, on the average. This can be expressed as (A+D) less than (B+C), and the center error CE=[(A+D)xe2x88x92(3+C)] less than 0. To correct this negative center error, the lens must be moved toward the center (ID) of the disk by the tracking control, once the sledge containing the photodiode assembly is moved toward the outside (OD).
In FIG. 4C, lens 12 is mis-aligned to the inside within the cavity of the sledge that includes photodiodes 10. A positive center error exists. Photodiodes A and D receive light reflected from region 16 on the optical disk, while photodiodes B and C receive the light reflected from region 14 on the disk. The middle of photodiodes 10 is shifted toward the outside of the optical disk compared with the middle of lens 12. The angle that light reflects from region 16 of the disk to photodiodes A, D is larger or more perpendicular when compared with the light reflected from region 14 to photodiodes B, C. Region 16 has a smaller area than region 14, so that the light intensity reflected from region 16 is larger. Thus the intensity of the light received by photodiodes A, D is greater than the intensity of light received by photodiodes B, C, on the average. This can be expressed as (A+D) greater than (B+C), and the center error CE=[(A+D)xe2x88x92(B+C)] greater than 0. To correct this positive center error, the lens must be moved toward the outside (OD) of the disk, which the tracking control loop does once the sledge with the photodiode assembly is moved toward the inside (ID).
What is desired is a sledge-center-error generating circuit. A circuit that receives inputs from quadrant photodiodes and detects a lens mis-alignment and generates a center-error signal is desired. A circuit that filters out high-frequency tracking errors is desired to generate a sledge-centering error signal. A feedback loop to control the position of the sledge is desired. More precise tracking is desired by adjusting the sledge position using a low-frequency sledge control loop.
A sledge-center-error generating circuit has photodiode inputs. It receives signals from photodiodes receiving a light beam reflected from a data layer of an optical disk. A first summer receives a first pair of the photodiode inputs. It generates a first sum signal.
A second summer receives a second pair of the photodiode inputs. It generates a second sum signal. A first subtractor receives the first sum signal and the second sum signal. It generates an overall error signal as a difference of the first sum signal and the second sum signal.
A high-pass element receives the overall error signal from the first subtractor. It stops low-frequency error signal and passes a high-frequency error signal.
A second subtractor receives the overall error signal and the high-frequency error signal. It generates a low-frequency error signal by non-linearly suppressing the high-frequency error signal from the overall error signal. The low-frequency error signal represents a sledge-center error. The low-frequency error signal for adjusting a position of a sledge containing the photodiodes. Thus the sledge-center error is generated by suppressing the high-frequency error signal.
In further aspects of the invention, the photodiode inputs are generated by a quadrant photodiode array. The first pair of photodiode inputs are on an outer side of the photodiode array and the second pair of photodiode inputs are on an inner side of the photodiode array. The inner side receives light reflected from an area of the optical disk with a smaller radius than an area reflecting light to the first pair.